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Annual Review of Marine Science - Volume 5, 2013
Volume 5, 2013
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The World Ocean Silica Cycle
Vol. 5 (2013), pp. 477–501More LessOver the past few decades, we have realized that the silica cycle is strongly intertwined with other major biogeochemical cycles, like those of carbon and nitrogen, and as such is intimately related to marine primary production, the efficiency of carbon export to the deep sea, and the inventory of carbon dioxide in the atmosphere. For nearly 20 years, the marine silica budget compiled by Tréguer et al. (1995), with its exploration of reservoirs, processes, sources, and sinks in the silica cycle, has provided context and information fundamental to study of the silica cycle. Today, the budget needs revisiting to incorporate advances that have notably changed estimates of river and groundwater inputs to the ocean of dissolved silicon and easily dissolvable amorphous silica, inputs from the dissolution of terrestrial lithogenic silica in ocean margin sediments, reverse weathering removal fluxes, and outputs of biogenic silica (especially on ocean margins and in the form of nondiatomaceous biogenic silica). The resulting budget recognizes significantly higher input and output fluxes and notes that the recycling of silicon occurs mostly at the sediment-water interface and not during the sinking of silica particles through deep waters.
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Using Triple Isotopes of Dissolved Oxygen to Evaluate Global Marine Productivity
L.W. Juranek, and P.D. QuayVol. 5 (2013), pp. 503–524More LessSince the triple isotopic composition of dissolved O2 (17Δ) was introduced as a natural tracer of photosynthetic gross O2 production (GOP) over 10 years ago, observations of 17Δ have been used to constrain marine productivity throughout the global ocean. This incubation-independent approach has several advantages: It allows the determination of production free from containment artifacts and reduces logistical hurdles that can make obtaining productivity with traditional incubation-dependent methods difficult. As such, GOP estimates derived from 17Δ have been used to give insight into potential biases in incubation-based approaches and to evaluate satellite-based estimates of production at the regional scale. With increased use, we have also learned more about the potential biases and uncertainties of this approach, some of which have been addressed by recent method improvements. We recap the major advances the 17Δ method has brought to improved understanding of biological carbon cycling, from incubation bottles to ocean basins.
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What Is the Metabolic State of the Oligotrophic Ocean? A Debate
Vol. 5 (2013), pp. 525–533More LessFor more than a decade there has been controversy in oceanography regarding the metabolic state of the oligotrophic subtropical gyres of the open ocean. Here we review the background of this controversy, commenting on several issues to set the context for a moderated debate between two groups of scientists. In one of the two companion articles, Williams et al. (2013) take the view that these gyres exhibit a state of net autotrophy—that is, their gross primary production (GPP) exceeds community respiration (R) when averaged over some suitably extensive region and over a long duration. In the other companion article, Duarte et al. (2013) take the opposite view, that these gyres are net heterotrophic, with R exceeding the GPP. This idea—that large, remote areas of the upper ocean could be net heterotrophic—raises a host of fundamental scientific questions about the metabolic processes of photosynthesis and respiration that underlie ocean ecology and global biogeochemistry. The question remains unresolved in part because the net state is finely balanced between large opposing fluxes and most current measurements have large uncertainties. This challenging question must be studied against the background of large, anthropogenically driven changes in ocean ecology and biogeochemistry. Current trends of anthropogenic change make it an urgent problem to solve and also greatly complicate finding that solution.
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The Oligotrophic Ocean Is Autotrophic*
Vol. 5 (2013), pp. 535–549More LessIn vitro observations of net community production (NCP) imply that the oligotrophic subtropical gyres of the open ocean are net heterotrophic; in situ observations, in contrast, consistently imply that they are net autotrophic. At least one approach must be returning an incorrect answer. We find that (a) no bias in in situ oxygen-based production estimates would give false-positive (net autotrophy) rates, (b) observed 13C enrichment of surface water dissolved inorganic carbon (DIC) can be explained only by positive NCP (net autotrophy), (c) lateral and vertical inputs of organic carbon are insufficient to sustain net heterotrophy, and (d) atmospheric input of organic material is too small to support in vitro rates of net heterotrophy and would yield δ13C depletion of surface DIC, quite the opposite of what is observed in the subtropical gyres. We conclude that the in vitro observations, implying net heterotrophy, must contain a bias that is due to an underestimate of photosynthetic rate and/or an overestimate of respiration rate.
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The Oligotrophic Ocean Is Heterotrophic*
Vol. 5 (2013), pp. 551–569More LessIncubation (in vitro) and incubation-free (in situ) methods, each with their own advantages and limitations, have been used to derive estimates of net community metabolism in the oligotrophic subtropical gyres of the open ocean. The hypothesis that heterotrophic communities are prevalent in most oligotrophic regions is consistent with the available evidence and supported by scaling relationships showing that heterotrophic communities prevail in areas of low gross primary production, low chlorophyll a, and warm water, conditions found in the oligotrophic ocean. Heterotrophic metabolism can prevail where heterotrophic activity is subsidized by organic carbon inputs from the continental shelf or the atmosphere and from nonphotosynthetic autotrophic and mixotrophic metabolic pathways. The growth of the oli-gotrophic regions is likely to be tilting the metabolic balance of the ocean toward a greater prevalence of heterotrophic communities.
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